Cacheable above one megabyte system management random access memory

ABSTRACT

Computer systems and methods that provide for cacheable above one megabyte system management random access memory (SMRAM). The systems and methods comprise a central processing unit (CPU) having a processor and a system management interrupt (SMI) dispatcher, a cache coupled to the CPU, and a chipset memory controller that interfaces the CPU to a memory. The memory includes system memory and the system management random access memory. The systems and methods un-cache the SMRAM while operating outside of system management mode, transfer CPU operation to system management mode upon execution of a system management interrupt (SMI), and change cache settings to cache the extended memory and system management random access memory with write-through. The systems and methods then change cache settings to cache the extended memory with write-back and un-cache the SMRAM upon execution of an resume instruction.

BACKGROUND

[0001] The present invention relates generally to computer systems and methods, and more particularly, to a system and method that provides for cacheable above one megabyte system management random access memory.

[0002] The assignee of the present invention develops firmware for computer systems known as a basic input and output system, or BIOS, along with other firmware and software that is employed in personal computer systems. In certain processors manufactured by Intel, for example, system management mode (SMM) is a special-purpose operating mode that is provided to handle system-wide functions such as power management, system hardware control, or proprietary OEM-designed code.

[0003] System management mode is intended for use by system firmware, not by applications software or general-purpose systems software. System management mode provides a distinct and easily isolated processor environment that operates transparently with respect to the operating system or executive and software applications.

[0004] When a processor is executing SMM code, it is said to be in SMM mode. At all other times the processor is executing foreground code in real or protected mode and is said to be in foreground mode.

[0005] When system management mode is invoked through a system management interrupt (SMI), the current state of the processor (the processor's context) is saved, and the processor switches to SMM mode in a separate operating environment contained in system management RAM (SMRAM). While in SMM mode, the processor executes SMI handler code to perform operations such as powering down unused disk drives or monitors, executing proprietary code, or placing the system in a suspended state.

[0006] When the SMI handler has completed its operations, it executes a resume instruction. The resume instruction causes the processor to reload its saved context, switch back to foreground mode, and resume execution of foreground code.

[0007] The processor handles an SMI on an architecturally defined “interruptible” point in program execution (which is commonly at an Intel Architecture instruction boundary). When the processor receives an SMI, it waits for all instructions to retire and for all stores to complete. The processor then saves its current context in SMRAM, enters SMM, and begins to execute the SMI handler.

[0008] The only way to exit from the system management mode is to execute the resume instruction. The resume instruction is only available in SMM mode. The resume instruction restores the processor's context by loading the state save image from SMRAM back into the processor's registers. It then returns program control back to the interrupted or foreground program code.

[0009] In conventional computer systems, executing code above one megabyte of system management random access memory (SMRAM) with a write-back cache enabled has the potential to destroy the data in system management random access memory area and hang the system. This is because cache line fills caused by reads and writes to addresses occupied by the system management random access memory area while executing code outside of system management mode (in foreground mode) may not actually be written back until a later time when executing code in system management mode.

[0010] Since reads to these memory locations that are made while executing code outside of system management mode fill the cache with unknown data (such as 0xFFFFFFFFh, for example), these values are later written in system management random access memory while dispatching a system management interrupt (SMI). Also if reads to the memory locations that have the SMI dispatcher codes while executing code outside of system management mode fill the cache with some unknown data, the system will hang in system management mode as the system management interrupt is generated. This means that a program, driver or virus that reads or writes these addresses could cause the system to fail or hang.

[0011] Executing code above one megabyte of system management random access memory with the write-back cache disabled is one potential solution that avoids corruption of the data above one megabyte of system management random access memory. However, this solution increases the latency of the system management interrupt, which may be more than a maximum acceptable latency of 500 microseconds.

[0012] Another option is to not use TSEG/HSEG chipset features and use a portion of system memory as the system management random access memory. However, this type of system management random access memory area is not secure while operating outside of system management mode.

[0013] A computer search of the US Patent and Trademark Office patent database was performed which uncovered a number of possible prior art patents. Patents uncovered in the search included U.S. Pat. No. 5,544,344, U.S. Pat. No. 5,638,532, U.S. Pat. No. 5,954,812, U.S. Pat. No. 5,596,741, and U.S. Pat. No. 5,475,829. A review of these patents reveals that these patents are generally unrelated to the specifics of the present invention.

[0014] It is therefore an objective of the present invention to provide for systems and method that implement a cacheable above one megabyte system management random access memory.

SUMMARY OF THE INVENTION

[0015] To accomplish the above and other objectives, the present invention comprises systems and methods that provide for cacheable above one megabyte system management random access memory. Exemplary systems and methods comprise a central processing unit (CPU) including a processor (or microprocessor), a system management interrupt (SMI) dispatcher (which is typically part of system firmware or basic input/output system (BIOS)), a level 1 (L1) cache, and a level 2 (L2) cache. The CPU is coupled by way of the bus interface and a bus to a chipset memory controller that interfaces to a memory. The memory comprises a lower memory (referred to as system memory), an upper memory, and an extended memory containing system management random access memory (SMRAM).

[0016] The systems and methods provide for cacheable above one megabyte system management random access memory as follows. The present invention secures the SMRAM while operating outside of system management mode (SMM). The present invention gains the benefit of caching by enabling TSEG/HSEG chipset features and performing various caching and un-caching operations. The TSEG/HSEG chipset features function to define the boundaries of the SMRAM.

[0017] The present invention un-caches the SMRAM while operating outside of system management mode (SMM). When an SMI is generated, the CPU operation is transferred to the system management mode. The SMI dispatcher changes cache settings to cache the extended memory and the SMRAM with write-through. The SMI dispatcher caches the extended memory with write-back and un-caches the SMRAM upon generation of a resume instruction (exit system management interrupt, or exit SMI) which exits the system management mode.

[0018] In operation, the system management mode (SMM) is invoked by a system management interrupt (SMI). The CPU informs the chipset that it is in SMM mode, and the chipset memory controller opens the SMRAM (including setting an AB segment, along with TSEG and HSEG segments). Thus, the chipset memory controller enables the CPU to access the SMRAM. The CPU saves the current state of the processor to SMRAM.

[0019] The CPU executes the SMI dispatcher (or SMI handler). In accordance with the present invention, the SMI dispatcher changes the caching setting as soon as possible to cache the extended memory and TSEGIHSEG with write-through. The SMI dispatcher then services the SMI event (i.e., the event that invoked SMI). Then, the SMI dispatcher changes the caching setting to cache the extended memory with write-back and un-caches the TSEG/HSEG

[0020] After the SMI handler or dispatcher has completed its operation, it executes a resume (RSM) instruction. This instruction causes the processor to reload the saved context of the CPU, switch back to non-SMM mode. The chipset memory controller is informed that CPU switched back to non-SMM mode, and it closes the SMRAM, so that the CPU cannot access SMRAM.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing, wherein like reference numerals designate like structural elements, and in which:

[0022]FIG. 1 illustrates a portion of an exemplary computer system in accordance with the principles of the present invention that provides for cacheable above one megabyte system management random access memory;

[0023]FIG. 2 illustrates an exemplary system memory of the computer system shown in FIG. 1; and

[0024]FIG. 3 is a flow diagram illustrating one exemplary method in accordance with the principles of the present invention that provides for cacheable above one megabyte system management random access memory.

DETAILED DESCRIPTION

[0025] Referring to the drawing figures, FIG. 1 illustrates a portion of an exemplary computer system 10 that provide for cacheable above one megabyte system management random access memory in accordance with the principles of the present invention. The computer system 10 includes, but is not limited to, a central processing unit (CPU) 11 that includes a processor or microprocessor (not shown), a system management interrupt (SMI) dispatcher 12 (which is typically part of system firmware or basic input/output system (BIOS)), a level 1 (L1) cache 13, illustrated as L1 data cache 13 a and L1 code cache 13 b, and a level 2 (L2) cache 15. The CPU 11 is coupled by way of the bus interface 14 and a bus 16 to a chipset memory controller 17 that interfaces to a memory 18.

[0026]FIG. 2 illustrates exemplary memory 18 employed the computer system 10 shown in FIG. 1. The memory 18 is arranged or partitioned to have a lower memory 21 (or system memory 21), an upper memory 22, and an extended memory 28 containing system management random access memory (SMRAM) 27. The memory 18 is typically greater than 1 megabyte in size.

[0027] In the exemplary system 10, the lower memory 21 or system memory 21 may be 640 kilobytes in size and the upper memory 22 may be 384 kilobytes in size, the size of extended memory 28 depends upon the number of DRAM chips that are used in the system 10. Video RAM space 23 is located in the upper memory 22 just above the 640 kilobyte lower memory 21 demarcation line.

[0028] Option read only memory (ROM) space 24 is allocated above the video RAM space 23. BIOS memory space 25 for the basic input/output system (BIOS) is allocated above the option ROM space 24. The size of video RAM space 23 plus the option ROM space 24 plus the BIOS memory space 25 is 384 kilobytes. The option ROM space 24 and the BIOS memory space 25 are hardware addressable.

[0029] The present invention provides for secure system management random access memory 27 while operating outside of system management mode (SMM). Thus, when using the present invention, the system management random access memory 27 is not destroyed when the system switches to system management mode (SMM).

[0030] The systems 10 and methods 30 provide for cacheable above one megabyte system management random access memory 27 as follows. The present invention secures the SMRAM 27 while operating outside of system management mode (SMM). The present invention gains the benefit of caching by enabling TSEG/HSEG chipset features in the chipset memory controller 17 and performing caching and un-caching operations. The TSEG/HSEG chipset features function to define the boundaries of the SMRAM 27.

[0031] Details regarding TSEG/HSEG chipset features are documented by Intel Corporation. In general, however, TSEG is a block of system memory (from [TOM-size of TSEG]:[TOM]) that is only accessible by the processor and only while operating in SMM mode. TOM stands for top of memory or top of low memory, the highest address of physical memory that below 4G. HSEG is a remap of the AB segment below 4G and above TOM (FEEA000:FEEBFFFF, for example),

[0032] Referring to FIG. 3, it is a flow diagram illustrating steps in implementing an exemplary method 30 in accordance with the principles of the present invention that provides for cacheable above one megabyte system management random access memory 27. While operating outside of system management mode (SMM), the SMRAM 27 is un-cached 31. When an SMI is executed 32, the CPU 11 was transferred to the system management mode and it executes SMI dispatcher 12. The SMI dispatcher 12 changes 34 cache settings to cache the extended memory 28 and SMRAM 27 with write-through. After the SMI event is serviced, the SMI dispatcher 12 changes 36 cache settings to cache the extended memory 28 with write-back and un-cache the SMRAM 27, then resume instruction is executed 35 which exits the system management mode.

[0033] Thus, and in operation, the system management mode (SMM) is invoked by a system management interrupt (SMI) 32. The CPU 11 informs the chipset memory controller 17 that it is in SMM mode, and the chipset memory controller 17 opens the SMRAM 27 (including setting an AB segment, along with TSEG and HSEG segments). Thus, the chipset memory controller 17 enables 33 the CPU 11 to access the SMRAM 27. The CPU 11 saves the current state of the processor to SMRAM 27.

[0034] The CPU 11 executes the SMI dispatcher 12 (or SMI handler 12). The SMI dispatcher 12 changes 34 the caching setting as soon as possible to cache the extended memory 28 and TSEG/HSEG with write-through. The SMI dispatcher 12 then services the SMI event (i.e., the event that invoked the SMI).

[0035] After the SMI dispatcher 12 has completed its operation, it changes 36 the cache settings to cache the extended memory 28 with write-back and un-cache the TSEG/HSEG (the SMRAM 27), and it executes 35 a resume (RSM) instruction. This instruction causes the processor to reload the saved context of the CPU 11, switch back to the non-SMM mode. The chipset memory controller 17 is informed that the CPU 11 has switched back to non-SMM mode, and it closes the SMRAM 27, so that the CPU 11 cannot access SMRAM 27.

[0036] Thus, systems and methods that provides for cacheable above one megabyte system management random access memory have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention. 

What is claimed is:
 1. A method for use in a computer system having a central processing unit (CPU) having a processor and a system management interrupt dispatcher, a cache coupled to the CPU, and a chipset memory controller that interfaces the CPU to a memory comprising system memory and system management random access memory (SMRAM), the method comprising the steps of: un-caching the SMRAM while operating outside of system management mode; transferring CPU operation to system management mode in response to execution of a system management interrupt; changing cache settings to cache the extended memory and system management random access memory with write-through; changing cache settings to cache the extended memory with write-back and uncache the SMRAM; and executing a resume instruction to exit SMM mode.
 2. The method recited in claim 1 wherein the central processing unit further comprises a level 1 cache.
 3. The method recited in claim 1 which further comprises a level 2 cache coupled to the central processing unit.
 4. The method recited in claim 1 wherein the system management interrupt dispatcher caches and un-caches the extended memory and the SMRAM by enabling predetermined TSEG/HSEG features of the chipset memory controller.
 5. The method recited in claim 4 wherein the TSEG/HSEG chipset features function to define boundaries of the SMRAM.
 6. A computer system comprising: a central processing unit (CPU) comprising a processor and a system management interrupt dispatcher; a cache coupled to the CPU; and a chipset memory controller that interfaces the CPU to a memory comprising extended memory and system management random access memory (SMRAM); wherein the system management interrupt dispatcher un-caches the SMRAM while operating outside of system management mode, transfers CPU operation to system management mode in response to a system management interrupt, changes cache settings to cache the system memory and system management random access memory with write-through, changes cache settings to cache the system memory with write-back and un-cache the SMRAM, and executes resume instruction to exit SMM mode.
 7. The computer system recited in claim 6 wherein the cache comprises a level 1 cache that is part of the central processing unit.
 8. The computer system recited in claim 6 wherein the cache further comprises a level 2 cache coupled to the central processing unit.
 9. The computer system recited in claim 6 wherein the system management interrupt dispatcher caches and un-caches the extended memory and the SMRAM by enabling predetermined TSEG/HSEG features of the chipset memory controller.
 10. The computer system recited in claim 9 wherein the TSEG/HSEG chipset features function to define boundaries of the SMRAM. 